Synapsis in a natural autotetraploid

To test assumptions of the autotetraploid chromosome pairing model regardin g events during synapsis, whole-mount spreads of synaptonemal complexes (SC s) of Machaeranthera pinnatifida (=Haplopappus spinulosus) (Asteraceae) (2n = 4x = 16) were analyzed by electron microscopy. On the assumption of one synaptic initiation per chromosome arm, each pachytene quadrivalent is expe cted to have one partner switch (PS), and the frequency of pachytene quadri valents for each chromosome is predicted to be 2/3 (or 0.67). However, to t he contrary, we observed a range of one to four PSs per pachytene quadrival ent with an overall mean of 1.56. This suggests that the number of synaptic initiations is greater than one per chromosome arm (or > two per chromosom e), and the predicted frequency of pachytene quadrivalents should be > 8/9 (based on a minimum of three initiations per chromosome). However, in close agreement with the model, the observed pachytene quadrivalent frequency fr om SCs in this study was 0.69. To explain the apparent discrepancy between the observed frequency of PSs and the observed frequency of quadrivalents, the possibility of nonindependent synaptic initiations and presynaptic alig nment are discussed in the context of their potential influence on quadriva lent frequency. Recombination nodules (RNs), which were scored in about hal f the SC spreads, occurred at a frequency (9.6 per nucleus) comparable with the chiasma frequency at diakinesis (9.3 per nucleus). The frequency of RN s as well as their distribution is consistent with the hypothesis that RNs occur at sites of crossing over and chiasma formation.